efficiency of flat plate heat exchanger

I may go with a Takagi Jr to heat my whole house and provide hot water (I can't go to wrong for a price of $585.00 for the Takagi Jr). Because my cast iron radiators must be isolated from the Takagi to keep ferrous elements from the magnetic flow switch of the Takagi, I must use a flat plate heat exchanger. Since I have to use this, I might as well pull my hot water off this also.

The Takagi Jr. has an efficiency of about 10% less than that of the most efficient boilers available, but for $600.00 and a life span of 20 years, that's fine with me.

Of concern to me is further efficiency loss through the heat plate exchanger. Does anyone know roughly what this loss is. Thanks., It will be a triangle tube TTP1-20E